Bypass ssh password prompt from a shell script

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http://unix.stackexchange.com – I've got a shell script that ssh'es into a server using public key authentication and runs a command on that machine. This works fine on machines where the users have installed the public key on the server. If this script is run from a machine where the user hasn't set up the authentication key, then the script will be stuck at password prompt. I want my script to carry on instead of getting stuck at the prompt. How can I bypass the prompt? I'm on a Mac OSX. The server is also running OSX (HowTos)